MR elements or heads can read data from a magnetic surface at high linear densities. An MR head detects magnetic field signals through resistance changes which are a function of the magnitude and direction of the magnetic flux sensed by the head.
U.S. Pat. No. 4,706,138 discloses a current biased, current sensing amplifier that biases and amplifies the signals produced by an MR element. The amplifier senses a current signal derived from the MR element which corresponds to dRh/Rh, where Rh is the resistance of the MR element and dRh is the magnetic-signal-induced change or delta in MR element resistance. This amplifier provides no short circuit protection and does not include means to maintain the MR element at a prescribed voltage (e.g., with reference to ground). It also employs so-called "bang-bang" feedback control and inputs a fixed signal approximating minimum offset error instead of changing the gain and linear response of the feedback loop.
U.S. Pat. No. 4,879,610 discloses an amplifier for biasing and amplifying the signals produced by an MR element. The amplifier amplifies a current signal derived from the MR element which corresponds solely to dRh, where dRh (as in U.S. Pat. No. 4,706,138) is the magnetic-signal-induced change in MR element resistance. This amplifier provides short circuit protection and is very suitable for use in applications involving high data rates provided the signal amplitude is relatively large. However, this amplifier provides an output with a relatively low signal-to-noise ratio. Signal-to-noise ratio becomes an increasingly serious problem in view of the trend in magnetic recording to continually increase track density and reduce signal amplitude.
U.S. Pat. No. 4,743,861 discloses a frequency compensation circuit for always increasing high frequency bandwidth when data rate is high, and under certain circumstances, such as in the circuit herein disclosed, when input lead inductance is high and input loop resistance is low.
None of these references or any other prior art known to applicants provides both short circuit protection and low noise. There is a need for a circuit that has (a) low noise to enable use in applications involving low amplitude input signals, (b) provides short circuit protection between the MR element and its environment, and (c) also varies the gain and frequency response of a feedback loop to enable faster switching between a plurality of MR elements.